The stock market can be daunting, especially during periods of volatility. But it's also a wealth-building powerhouse, and investing remains one of the easiest and most effective ways to make a lot of money over time.

You also don't need to be a market expert to earn hundreds of thousands of dollars or more. In fact, there's one investment that can potentially turn $100 per month into more than $197,000 over time -- and it requires next to no effort on your part.

A safe yet powerful investment

One of the safest and most effortless investments out there is the S&P 500 ETF. This type of fund tracks the S&P 500 itself and contains the same stocks as the index, which are from 500 of the largest and most stable companies in the U.S.

Because it's impossible to invest in the actual index, an S&P 500 ETF -- such as the Vanguard S&P 500 ETF (VOO 0.62%) or iShares Core S&P 500 ETF (IVV 0.64%) -- mirrors its performance and aims to earn similar returns over time.

The S&P 500 has a fantastic track record of earning positive long-term returns. In fact, it's actually harder to lose money with this investment over the long run than it is to make money.

Analytics firm Crestmont Research analyzed the S&P 500's rolling 20-year total returns starting in 1900. Researchers found that in all 104 periods (from 1919 to 2022), the index earned positive 20-year total returns.

This means that if you had invested in an S&P 500 ETF or index fund at any point after 1900 and held it for 20 years, you'd have earned positive returns -- regardless of how volatile the market was during that time.

Building a $200,000 portfolio

Perhaps the best advantage of this type of investment, though, is that it's incredibly low-maintenance. S&P 500 ETFs are passive investments, which means you don't need to actively manage your portfolio by choosing stocks or deciding when to buy or sell.

All you have to do, then, is invest consistently and give your money as much time as possible to grow. The longer you let your money sit, the more you'll earn over time.

Historically, the S&P 500 itself has earned an average rate of return of around 10% per year, meaning its annual highs and lows have averaged out to roughly 10% per year over decades.

Assuming you're investing $100 per month and earning a 10% average annual return, here's approximately how much you could accumulate over time, depending on how many years you invest:

Number of Years Total Savings
20 $68,700
25 $118,000
30 $197,400
35 $325,200
40 $531,100

Source: Author's calculations via Investor.gov

To accumulate close to $200,000 total, you'll need to invest $100 per month for around 30 years. But if you have more time to invest, you could earn exponentially more. And if you can afford to invest more than $100 per month, that will also supercharge your savings.

For example, if you were to invest $200 per month, all other factors remaining the same, you'd have earned roughly $395,000 after 30 years. Invest $500 per month, and you'd have around $1 million in that timeframe.

There's not necessarily a right or wrong way to invest, and your strategy may differ based on your goals and risk tolerance. But the S&P 500 ETF can be a fantastic way to limit risk while still earning a significant amount over time, and the sooner you get started, the more you can potentially earn.